The positron emission tomography scanner (PET) is a nuclear-medicine imaging equipment utilizing a positron emission nuclide, for use in various applications such as diagnosis of cancers and molecular imaging.
The positron emission nuclide is an isotope such as 18F which is unstable due to an excessively large number of protons in an atomic nucleus as compared with a number of neutrons, having the radioactivity of emitting positrons and neutrinos in association with β+ decay. The thus emitted positron, the antimatter of electron, undergoes pair annihilation when colliding with an electron and the mass of the positrons and electrons is all converted to energy. This energy is radiated in a form of high-energy electromagnetic waves which is called annihilation radiation. Because of the law of conservation of momentum before and after pair annihilation, mainly two annihilation radiation photons are emitted at the same time and approximately in the opposite direction. Strictly speaking, although there is a case where a single photon or three or more photons are emitted, the percentage is less than 1% of the total and, therefore, to be negligible in the PET imaging. Where two photons are emitted, each of the energies corresponds to the mass of one (positron) electron, that is, approximately, 511 keV.
The principle of imaging is the coincidence of annihilation radiation. Where radiation of 511 keV is determined substantially at the same time by two mutually opposing radiation detectors, it is most likely that positrons have undergone pair annihilation on a straight line connecting these two radiation detectors. This information is collected, as shown in FIG. 1(A), by using many radiation detectors 16 arranged around a scanned substance 10 and reconstructed by a mathematical approach similar to that used in an X-ray CT. Thereby, it is possible to obtain a tomographic image similar to the distribution of a positron emission nuclide 12 in the test substance 10. In this drawing, the numeral 18 depicts a bed.
Therefore, a performance required for the radiation detector 16 is to determine an incident position, energy and incident time of the annihilation radiation 14 as accurately as possible. Here, substantially at the same time means, in general, time within 15 nanoseconds (nano is a prefix which denotes a factor of 10−9), where a radiation detector is able to determine time more accurately, the time is to be less than 10 nanoseconds or less than 5 nanoseconds. If a time frame (coincidence time window) for judging incidence of two annihilation radiation photons to be one annihilation radiation set generated from one pair annihilation, and regard as being at the same time is shortened, it is less likely that a plurality of annihilation radiation photons resulting from different pair annihilation are mistakenly combined. Therefore, determination accuracy can be enhanced to improve a signal-to-noise ratio. In addition, the above-described combination of annihilation radiation photons detected by determining a time window is referred to as coincidence counting or coincidence.
It is known that where a capacity of processing an electric signal from the radiation detector 16 to determine the incident time of the annihilation radiation 14 is, in general, less than 1.5 nanoseconds, time-of-flight (TOF) of annihilation radiation is utilized, with the time window kept in a range which will not cause counting leakage in a correct combination of annihilation radiation photons, thereby improving a signal-to-noise ratio in a positron emission tomography (PET) scanner. For example, where pair annihilation takes place at the centers of two opposing radiation detectors, two annihilation radiation photons arrive at the radiation detectors at the same time. Further, where pair annihilation takes place at a coordinate (spatial coordinate) closer to one of the radiation detectors, annihilation radiation will arrive earlier at the closer radiation detector. In other words, a difference between the arrival time at one radiation detector and that at the other radiation detector is determined, it is possible to convert the time difference to a spatial difference between a distance from a spatial coordinate where pair annihilation takes place to one radiation detector and a distance from there to the other radiation detector. In a PET scanner which does not utilize conventional time-of-flight as shown in FIG. 1(A), information obtained from one set of coincidence is a straight line including a spatial coordinate where pair annihilation is considered to have taken place. However the use of time-of-flight makes it possible to narrow down to a certain region on the straight line like a time-of-flight type PET (TOF-PET) scanner shown in FIG. 1(B). The narrowing-down accuracy is determined depending on the time resolution of the scanner concerned. As the determination accuracy is increased, information on a position of pair annihilation is increased to result in enhancement of a signal-to-noise ratio (refer to IEEE Trans. Nucl. Sci., Vol. 50, No. 5, pp. 1325-1330, 2003, by W. W. Moses). Therefore, it is preferable that radiation detectors to be loaded on the TOF-PET have higher time determination accuracy.
In addition, where the capacity for determining the incident time of annihilation radiation is, in general, less than 100 picoseconds all over the scanner (pica is a prefix denoting a factor of 10−12), not only a signal-to-noise ratio but also the spatial resolution of a tomographic image is expected to be enhanced. Technology for enhancing the time resolution of radiation detectors has been strongly requested.
The concept of the TOF-PET scanner utilizing difference of the time-of-flight of annihilation radiation has been known since the 1980s (refer to IEEE Trans. Nucl. Sci., Vol. 28, No. 6, pp. 4582-4589, 1981 by T. Tomotani). However, at the technical level at that time, a scintillator and a radiation detector used as a radiation detecting element as well as a circuit for processing electric signals from the radiation detector were insufficient in performance and others, and therefore no improvement was made in a signal-to-noise ratio. At the present time, scintillators excellent in response speed such as LSO (lutetium silicate to which a small quantity of cerium is added) and LYSO (a mixed crystal of LSO with yttrium silicate to which a small quantity of cerium is added) have been developed. Further, a photomultiplier tube (PMT) used as a light receiver for detecting scintillation light caused by interaction with radiation is also improved in time determination accuracy. Since integrated circuit technologies for specific uses are also enhanced, it has been confirmed that a TOF-PET scanner utilizing difference of the time-of-flight of annihilation radiation is superior to a conventional PET scanner in performance of a signal-to-noise ratio. Therefore, there is strong demand for a radiation detector more excellent in time resolution. When the signal-to-noise ratio is enhanced, it is possible to shorten the time necessary for positron emission tomography and decrease the dosage of a radiopharmaceutical to be administered to a subject.
As shown in FIG. 2, a first error parameter of detection time is caused by a difference in transfer speed between annihilation radiation 14 and scintillation light 24 within a scintillator 22. In the drawing, the numeral 20 depicts a light receiver such as a photomultiplier tube.
The flight speed of the annihilation radiation 14 is equal to light speed c under a vacuum (approximately 300,000 km per second) either under a vacuum or in a medium. On the other hand, the scintillation light 24 is approximately equal to c in flight speed in the atmosphere but reduced in speed to c/n in a scintillator. Here, n denotes a refractive index of the scintillator and, in general, a value greater than 1.0.
Since annihilation radiation at 511 keV is greater in penetration force, a scintillator 22 having the thickness of a few centimeters is, in general, used for effective detection.
As shown on the right side in FIG. 2(A), where the annihilation radiation 14 interacts with the scintillator 22 in the vicinity of an upper end of the scintillator 22, the scintillation light 24 needs to fly at a long distance inside the scintillator 22 until arriving at the light receiver 20. On the other hand, as shown on the left side in FIG. 2(A), where it interacts therewith in the vicinity of a lower end of the scintillator 22, the scintillation light 24 will fly at a short distance until arriving at the light receiver 20. In other words, as shown in FIG. 2(B), apparent detection time is made earlier in a case where the annihilation radiation 14 flies at a long distance inside the scintillator 22, with the flight speed c kept, and the light is converted to the scintillation light 24 with the flight speed c/n immediately before the light receiver 20.
Here, where in determining one annihilation radiation set respectively by two radiation detectors, scintillation light is generated in the vicinity of an upper end of a scintillator at one detector and scintillation light is generated in the vicinity of a lower end of the scintillator at the other detector, a spatial coordinate of pair annihilation expected by a difference in the detection time is made closer to the other radiation detector than in actuality. Therefore, if a difference in the detection time resulting from a difference in transfer speed between annihilation radiation and scintillation light in the scintillator is corrected, it is possible to increase the information accuracy of difference of the time of flight. In addition, in FIG. 2(A), in order to simplify the principle, one scintillation photon is representatively emitted directly below per annihilation radiation. In actuality, several thousands or tens of thousands of photons are emitted, and a direction in which they are emitted is not necessarily limited to being directly below. Further, since some of the photons are absorbed by a boundary surface of the scintillator, a reflective material and others, all the photons do not necessarily arrive at a light receiver.
As illustrated in FIG. 3(A) a second error parameter of detection time is caused by a difference in the distance of a channel where the scintillation light 24 is transmitted through the scintillator 22. The scintillation light 24 is partially made incident directly into the light receiver 20, however, in general, more than half of the photons are reflected more than once on an upper surface or side surfaces of the scintillator 22 and then made incident into the light receiver 20. For example, as shown on the right side in FIG. 3(A), the scintillation light 24 that generated in the vicinity of an upper end of the scintillator 22 and radiated above arrives immediately at the upper end of the scintillator 22 and goes downward by being reflected by a reflective material and others covering the upper surface of the scintillator 22. On the other hand, as shown on the left side in FIG. 3(A), the scintillation light 24 radiated upward from the vicinity of a lower end of the scintillator 22 flies by a length of the scintillator 22 until being reflected downward at the upper end of the scintillator 22. Further, where the scintillation light 24 is emitted, with an angle kept in a lateral direction, a transmission channel will change by reflection on side surfaces of a scintillator. Further, where scintillators are arrayed in all directions two- or three-dimensionally, reflection or refraction among the scintillators also allows the transmission channel to change. As the transmission channel is made longer, the scintillation light 24 takes a longer time accordingly for arriving at the light receiver 20, and the time for determining the detection of annihilation radiation is delayed.
FIG. 3(B) shows a result in which LSO crystals, each of which measures 1.45 mm×1.45 mm×4.50 mm, are arrayed in a square shape by 32 pieces×32 pieces, which is prepared as one stage (layer), and in order to detect a position at which annihilation radiation interacts with scintillator crystals at higher accuracy, the thus prepared stage is stacked in four stages to give a crystal block, and based on this crystal block, calculation has been made for a transmission channel of scintillation light and the transmission time thereof. Here, a first layer in the drawing is closest to a radiation source and a fourth layer is furthest from there. In order to show the principle simply, a relationship between the number of photons arriving at a light receiver and elapsed time where one hundred thousands of photons are emitted at a reference time in random directions from the center of each crystal which has been selected at the center of each stage is shown. It is apparent therefrom that not only is the arrival time of a first photon different depending on a distance of a straight line between a light-emitting coordinate inside a scintillator crystal which has emitted light and a light receiver, but also there are times when the number of arriving photons is greatest and a second peak resulting from reflection on an upper surface of the crystal. If the thus generated difference in detection time due to a difference in the distance of a transmission channel of scintillation light is corrected, it is possible to increase the information accuracy of difference of the time of flight.
A third error parameter of the detection time is a difference in the output wave of a light receiver caused by a difference in the transmission channel. As apparent from FIG. 3(B) showing the time distribution of light (input) arriving at the light receiver, time necessary from arrival of the first photon from each layer to the time when the number of arriving photons is greatest is different depending on each layer. It is also apparent from the shape of the graph that the trend of increasing the number of photons with the lapse of time is different. In order to determine the time from an output wave of the light receiver in the simplest way, first, a threshold value is set to avoid confusion of noises with a signal, an output in excess of the threshold value is regarded as a signal, and time in excess of the threshold value is given as detection time. If the definition is made common in all the light receivers, there will be no difference resulting from a definition method. However, in this method, as shown in FIG. 3(C), where an output signal is relatively large (for example, a fourth layer), the threshold value is exceeded soon after arrival of a first photon, but where the output signal is relatively small (for example, a first layer), the threshold value is exceeded only around the time when the output is maximum. Therefore, the time to be determined is deviated depending on a magnitude of the signal. Therefore, in actuality, a more sophisticated determination method such as a constant fraction method in which time is not deviated depending on a magnitude of the output signal is extensively used (refer to Radiation Handbook, third edition, pp. 753, 2001, published by Nikkan Kogyo Shimbun).
However, since even the constant fraction method capable of coping with the change in magnitude of an output signal is unable to cope with the change in waveform of the output signal, a deviation of time which is determined by a difference in whether a signal rises abruptly or slowly is caused. Thus, if correction is made for a difference in the detection time which is caused by a difference in the output waveform of a light receiver resulting from a different transmission channel of scintillation light, it is possible to increase the information accuracy of difference of the time of flight. This is effective not only in the constant fraction method but also in a leading edge method or other determination-making methods. A difference can be corrected, for example, on the basis of a gradient found when a signal rises or the change thereof.
Technology for correcting detection time by utilizing only information on a light emitting position in the depth direction by the use of a radiation detector as shown in FIG. 4, in place of information on a three-dimensional position (light emitting position) is already known (refer to IEEE Trans. Nucl. Sci., Vol. 53, No. 1, pp. 35-39, 2006, by T. Tsuda et al.). In the drawing, the numeral 40 depicts a radiation detector (referred to as a DOI detector) which is capable of obtaining information on depth of interaction (DOT) and made up of, for example, a 256-channel position-sensitive-type photomultiplier tube (PS-PMT) 21 and a scintillator crystal block 23 stacked, for example, in four layers by 6×6 proposed by the applicant in Japanese Published Unexamined Patent Application No. 2004-279057.
According to the above-described DOI detector, it is possible to obtain information on depth of interaction.
However, the DOT detector stacked in four layers as shown in FIG. 4 has following problems. Namely, a first layer on the side of a radiation source is greatest in TOF difference (delay) and lowest in time resolution due to a long distance to the PS-PMT 21 withal the fact that the number (frequency) of light emitting events by incident radiation is greatest. On the other hand, a fourth layer on the side of a light receiver is smallest in TOF difference (delay) and highest in time resolution due to the nearest distance to the PS-PMT 21, but the fourth layer is not effectively utilized because the number of events is smallest.
FIG. 5 shows the respective responses of four layers in a conventional DOI detector which is not subjected to TOF correction and time resolution of the detector as a whole. In this example, the time resolution (half bandwidth FWHM) of the radiation detector as a whole was 361.4 ps.
On the other hand, where TOF correction was made in every layer, as shown in FIG. 6, the time resolution of the radiation detector as a whole was improved to be 324.1 ps. However, this was still insufficient.